The DRM (Digital Right Management) systems that manage copying and playback of digital video and audio contents for the copyright protection by imposing limitations on the copying and playback are becoming increasing popular.
Some standards have been established for the DRM system. The standards include OMA DRM R2 for mobile phones and Marlin for digital appliances. In the DRM standards, contents are associated with right information that imposes various limitations on the playback rights, such as the maximum playback time for which the contents can be played, the maximum playback counts for which the contents can be played, and the expiration deadline of the playback. DRM-compliant devices are required to adhere to the right information in order to perform processing for playing back the contents and consuming the right information.
The DRM standards must be implemented to guarantee the prevention of unauthorized playback, which infringes the prescribed playback right by tampering with the right information or bypassing the consumption processing. For example, it is necessary to prevent a malicious user from circumventing the consumption by cutting off a power supply to a playback device deliberately in advance of the consumption processing.
Patent Literature 1 discloses a method for monitoring whether unauthorized processing has occurred during the playback of contents by monitoring whether proper operations are performed in specific command processing at regular time intervals.
Among the DRM standards, Marlin is characterized by its callback processing for executing prescribed operations if any one of prescribed conditions is satisfied. Specifically, the callback processing is for performing operations, such as playback interruption, advertisement display, and charging, when the accumulated playback time is greater than the prescribed maximum playback time. Non-Patent Literature 1 discloses several types of the callback processing. In the Marlin DRM system, various types of the right information using the callback processing are specified, whereby a wider variety of the playing back processing is supported compared with conventional DRM systems.